Constraining Cosmology with Double-Source-Plane Strong Gravitational Lenses From the AGEL Survey

Abstract: Double-source-plane strong gravitational lenses (DSPLs), with two sources at different redshifts, are independent cosmological probes of the dark energy equation of state parameter $w$ and the matter density parameter $\Omega_{\rm m}$. We present the lens model for the DSPL AGEL035346$-$170639 and infer cosmological constraints from this system for flat $\Lambda$CDM and flat $w$CDM cosmologies. From the joint posterior of $w$ and $\Omega_{\rm m}$ in the flat $w$CDM cosmology, we extract the following median values and 1$\sigma$ uncertainties: $w = -1.52^{+0.49}_{-0.33}$ and $\Omega_{\rm m} = 0.192^{+0.305}_{-0.131}$ from AGEL0353 alone. Combining our measurements with two previously analyzed DSPLs, we present the joint constraint on these parameters from a sample of three, the largest galaxy-scale DSPL sample used for cosmological measurement to date. The combined precision of $w$ from three DSPLs is higher by 15% over AGEL0353 alone. Combining DSPL and cosmic microwave background (CMB) measurements improves the precision of $w$ from CMB-only constraints by 39%, demonstrating the complementarity of DSPLs with the CMB. Despite their promising constraining power, DSPLs are limited by sample size, with only a handful discovered so far. Although ongoing and near-future wide-area sky surveys will increase the number of known DSPLs by up to two orders of magnitude, these systems will still require dedicated high-resolution imaging and spectroscopic follow-ups like those presented in this paper. Our ASTRO 3D Galaxy Evolution with Lenses (AGEL) collaboration is undertaking such follow-up campaigns for several newly discovered DSPLs and will provide cosmological measurements from larger samples of DSPLs in the future.